[Rivet] MSG_DEBUG in plugins

[Holger Schulz]

> It's that those MSG_DEBUG functions need to get a named logger object. 
> Analyses and Projections automatically have one of those, accessed via 
> their getLog method, so MSG_DEBUG etc. will work there. If you make 
> this a member function (automatically inline if written as usual) 
> rather than a standalone function, then it'll work.
>
> Andy
>
Cool, thanks!

Holger